Immnuoregulatory Effect of Human Mesenchymal Stem Cell Is Mediated Via STAT3 Molecule

The best curative treatment modality in hematologic malignancies is allogeneic hematopoietic stem cell transplantation (HSCT). Graft-versus-host disease (GVHD) is a major obstacle of allogenic HSCT. Bone marrow derived human mesenchymal stem cells (MSCs) is known to have immunoregulatory effect in vitro and in vivo via inhibiting alloreactive T lymphocytes, leading to their clinical use for the prevention of GVHD in HSCT. However, the molecular mechanism of immunoregulatory effect of human MSCs is not fully understood. In this study we investigated the signaling of immunoregulatory effect by co-culture of human MSCs with lymphocytes. The proliferation of allogeneic T cells was strongly inhibited. The fraction of CD4⁺CD25⁺Foxp3⁺ cells (Treg cells) was increased, while the fraction of CD4⁺, CD8⁺, CD25⁺ was decreased. In addition, induction of Th1 to Th2 shift was observed. Western blot study showed that phosphorylation of STAT1, STAT3, STAT6 was up-regulated, but STAT1, STAT4, ERK, AKT, NF-κB (p65, p50 subunits) was down-regulated. While expression of STAT3 was observed in culture of MSCs only, no expression of STAT3 was shown in co-cultured human MSCs with lymphocytes. In order to validate our results, expression of STAT3 in human MSCs co-cultured with lymphocytes was ablated using small interfering RNA. As a result, inhibition of CD4⁺CD25⁺FoxP3⁺ cells, proliferation of allogenic T lymphocytes, inhibited induction of Th1 to Th2 shift and proliferation of CD4⁺, CD8⁺, CD25⁺ cells. Furthermore, expressions of Th1 and Th17-related cytokines were increased, while expressions of Th2-related cytokines were decreased. In summary, these results suggest that STAT 3 may be an indispensable molecule in the immunoregulatory effects in human MSCs via modulation of regulatory T cells.